F.P. van der Meer

Continuum Models for the Analysis of Progressive Failure in Composite Laminates

The performance of continuum material models for mesolevel modeling of progressive failure in composite laminates is examined. Two different continuum models are used: a continuum damage model and a softening plasticity model. It is shown how mesh-independent results can be obtained with both models by introducing a viscosity term. The capability of the models to represent complex intraply failure behavior is assessed by means of an analysis of a notched plate, where interface elements are used to model delamination. Proper results from different modeling approaches are shown, but upon calibration a limitation of the continuum approach in the representation of matrix failure is encountered. With a second example, this limitation is emphasized further and explained as a consequence of the homogenization that is inherent in continuum models, irrespective of the applied failure criteria and material degradation laws.

Computational modeling of complex failure mechanisms in laminates

A computational framework for the simulation of progressive failure in composite laminates is presented. The phantom-node method (a variation to the XFEM) is used for a mesh-independent representation of matrix cracks as straight discontinuities in the displacement field. Furthermore, interface elements are used for delamination and a continuum damage model for fiber failure. The framework is validated against experimental observations for open-hole tests and compact tension tests. It is shown that different failure mechanisms are captured well, which allows for the prediction of size effects.

The influence of curing cycle and through thickness variability of properties in thick laminates

Mechanical properties of glass fibre reinforced polymers are dependent on the manufacturing curing cycles. During the laminate manufacturing process, each thickness position experiences a different local curing cycle. Therefore, it can be expected that mechanical properties vary through the thickness, particularly for thick laminates. To study the through-thickness variation of static and fatigue mechanical properties, thick laminates were divided into sub-laminates and these sub-laminates were separately tested. The present work reports temperature profiles through the thickness recorded during the manufacturing of thick laminates, as well as experimental data from static and fatigue tests (S–N curves) of sub-laminates obtained at different thickness positions. The variation of the mechanical properties through the thickness is discussed and related to the local curing temperatures experienced by each sub-laminate.

Experimental/numerical study of anisotropic water diffusion in glass/epoxy composites

In this work a glass/epoxy composite commonly used in wind turbine blades is exposed to a humid environment at an elevated temperature. To research the anisotropic diffusion behaviour observed in unidirectional composite specimens, experimental results of slices cut along the three directional planes of the laminate immersed in demineralised water at 50° C are coupled with numerical modelling. The weight of the slices was measured at regular intervals, from which the uptake behaviour could be deduced. The process was modelled using a 3-dimensional RVE of the material, where diffusion is modelled as steady-state and the diffusivity in each direction was measured by applying concentration gradients to the model. The experimental data shows similar water uptake behaviour for samples in both transverse directions, while the water uptake in the fibre direction was significantly faster. A proper fit according to Ficks law was obtained for the transverse direction, while this was not possible for the samples in fibre direction, suggesting a strong dependency of the diffusion behaviour on the fibre orientation. Results from the proposed numerical models show that the geometric effect of fibres acting as barriers for the water movement is indeed responsible for part of the observed anisotropy.

Interaction Between Intraply and Interply Failure in Laminates

A mesoscale model for finite element analysis of failure in laminates is presented. The model consists of separate parts for failure inside a ply (intraply) and failure between plies (interply). Both parts offer a description from onset of failure to complete local failure, thus allowing for progressive failure analysis. Intraply failure is simulated with a softening plasticity model based on a Tsai-Wu criterion with viscoplastic regularization. Details are presented on the implementation of the softening law for orthotropic materials in finite element computation. Interply failure is modeled using interface elements with a damage law for mixed mode delamination. The performance of the model is illustrated by means of an analysis of a laminate with a sharp internal notch – a case in which different modes of ply failure successively take place and interact with failure between the plies.

A level set model for delamination in composite materials

Abstract In this chapter, we present a new approach to delamination growth modeling where the delamination front is described with the aid of the level set method. The extended finite element method is used at the front to enrich the elements. The discontinuity in stress and strain that results from the enrichment is used for computation of energy release rates. This approach is more efficient than cohesive zone methods because large elements and large time steps can be used. In contrast with other fracture mechanics methods, arbitrary non-self-similar crack growth can be represented smoothly. The method is demonstrated with three-dimensional analysis of a double cantilever beam.

Simulation of Progressive Failure in Composite Laminates

Fiber reinforced polymers are materials with excellent mechanical properties and relativelymuch design freedom.However, because complex failuremechanisms originating from the microstructure of the material may occur, realistic simulation of the failure process is still a challenge. Two alternative models for the modeling of failure in composite laminates are presented. The first is a continuum damage model that is supposed to cover all ply failure mechanisms. A limitation of the continuum approach with respect to the modeling of matrix failure is illustrated.Therefore, a discontinuousmodel has been developed for matrix failure specifically